1. Field of the Invention
The present invention is directed to data networks. More particularly, the present invention is directed to an interface and method for transferring data over a data network, such as an access network.
2. Background
The term “access network,” as used herein, refers to a data network that connects a service provider to multiple end users, such as individuals, businesses and other organizations, for the delivery of voice, video and data services. Within an access network, the transfer of data from a service provider to the end users may be generally described as “downstream” communication and, conversely, the transfer of data from the end users to the service provider may be generally described as “upstream” communication. In conventional access networks, the service provider typically also provides end users with a portal to one or more other data networks, such as a wide area network or metropolitan area network.
Various technologies have been used to deliver data over access networks. For example, the two technologies most commonly used today rely on physical links that have existed for many decades. The oldest medium is twisted pair, which includes the standard copper wire that connects most homes to a local telephone company. Sophisticated technology has been used to deliver data over long twisted pair loops at speeds of 1 Mbps (megabit per second) and over shorter lengths at rates up to tens of Mbps. The other commonly-used medium is coaxial cable, upon which data is transferred using specially-designed cable modems. This technique permits data rates up to approximately 100 Mbps. However, because of the broadcast nature of cable modem networks, this capacity must be shared between a number of active users. For example, as many as 50 or more users may be required to share this bandwidth in a cable modem system.
Extending these two traditional methods to permit higher data rates would require considerable effort. Thus, if higher data rates are desired, other technologies must be explored. A technique that is now being widely considered is to implement an access network using Ethernet technology. Because Ethernet technology has been extensively developed for use in enterprise networks such as Local Area Networks (LANs), Ethernet systems and chipsets now comprise the most inexpensive link layer technology available. Moreover, it has been observed that Ethernet technology may be advantageously used in access networks that include fiberoptic cable to provide higher transmission rates over longer distances.
One possible architecture for an Ethernet-based access network involves providing multiple point-to-point connections through the installation of a dedicated fiber optic cable between the service provider facility and each end user location. Such an architecture provides a number of advantages. For example, because a network of dedicated fiber optic cables may consist entirely of passive elements, very little network maintenance is required. This is in contrast to other types of access networks, such as a hybrid-fiber coaxial (HFC) network, which includes many active elements (such as filters and amplifiers) that must be periodically tested and adjusted to maintain system performance. Furthermore, in a network of dedicated fiber optic cables, system upgrades may be achieved by simply modifying the electronics at the service provider facility and/or the end user locations, while avoiding modification of intervening components. Unfortunately, the cost of deploying such an architecture may be prohibitively expensive, as it requires the installation of very large amount of fiber optic cable before service may be provided.
An alternate architecture for an Ethernet-based access network includes at least one active element relatively close to the home (e.g., about a kilometer to the home). The active element is an Ethernet switch that provides a point-to-multipoint interface between a single link to the service provider facility and a plurality of dedicated cables each of which is connected to a different end user location. For example, FIG. 1 shows a portion of an access network 100 in which an Ethernet switch 102 is utilized to transfer data packets between a plurality of end-user devices 104a–104n and a higher level node 110 within the access network. The Ethernet switch 102 is connected to the end user devices 104a–104n through copper or fiber optic cables 106a–106n, respectively, and is connected to the higher level node 110 via a single fiber optic cable 108. For the purposes of this example, it will be assumed that the higher level node 110 is the service provider facility; however, the higher level node 110 may also be another interface within the access network, such as a hub, an optical node, or another Ethernet switch.
This alternate architecture is less expensive than the previously described Ethernet-based architecture, as it requires the installation of considerably less cable. For example, as shown in FIG. 1, only a single fiber optic cable 108 is required to connect the service provider facility to the Ethernet switch 102, while dedicated cables 106a–106n are required only for the short distance between the Ethernet switch 102 and the end user locations. Furthermore, this alternate architecture is advantageous because it permits the upstream link from the end user devices 104a–104n to the Ethernet switch 102 to be operated at a lower transmission rate than the upstream link from the Ethernet switch 102 to the higher level node 110. This is because the upstream bandwidth requirements for each end user device 104a–104n are considerably less than that of the Ethernet switch 102, which must accommodate the combined upstream bandwidth requirements of all the end user devices 104a–104n. Because the upstream link from the Ethernet switch 102 to the end user devices 104a–104n may be run at a lower transmission rate, cheaper components may be used to implement the cables 106a–106n and the end user devices 104a–104n. 
It has been observed, however, that conventional Ethernet devices, such as the conventional Ethernet switch 102, are not ideally suited for providing a point-to-multipoint interface within an access network. For example, conventional Ethernet switches provide costly switching functionality that may provide little benefit in an access network. Additionally, conventional Ethernet switches transmit data at the same rate from all ports, and therefore are not well-suited for access networks in which bandwidth requirements may be different in the upstream and downstream direction. These concepts may be best explained with reference to FIG. 2, which depicts an example implementation of the Ethernet switch 102 described in reference to FIG. 1 above.
As shown in FIG. 2, Ethernet switch 102 includes a plurality of transceivers 202a–202n for transmitting and receiving data over downstream links 106a–106n, respectively. Data received over downstream links 106a–106n may be temporarily queued in a corresponding receive buffer 206a–206n, while data to be transmitted over downstream links 106a–106n may be temporarily queued in a corresponding transmit buffer 204a–204n. Ethernet switch 102 also includes a transceiver 218 for transmitting and receiving data over upstream link 108. Data received over upstream link 108 may be temporarily queued in a receive buffer 216 and data to be transmitted over upstream link 108 may be temporarily queued in a transmit buffer 214.
In accordance with the example implementation of FIG. 2, Ethernet packets received on any upstream or downstream link may be routed for transmission over any other link, or for simultaneous transmission over any combination of other links. For example, an Ethernet packet may be received and queued in any one of receive buffers 206a–206n or receive buffer 216. Control logic 210 is configured to schedule the serving of packets in these receive buffers according to a predetermined algorithm. For example, the control logic 210 may serve a packet at the head of a queue within each receive buffer, rotating sequentially in a round-robin fashion among the buffers. In accordance with this algorithm, the control logic 210 examines the header of each Ethernet packet and determines if it can be served. If it cannot be served, the control logic 210 passes to the next receive buffer in turn and attempts to serve the packet at the head of its queue.
When a packet is ready to be served, routing logic 212 examines the header of the Ethernet packet and, based on the destination address within the packet header, determines which link to route the packet to for transmission and routes the packet to the appropriate transmit buffer servicing that link. The destination link may be any of the output links 106a–106n or 108, or any combination of these links. The switching fabric 208 provides the physical interconnection between the receive buffer and each appropriate transmit buffer. Once the Ethernet packet has been transferred to an appropriate transmit buffer via the switching fabric 208, it may then be transmitted out of the Ethernet switch 102.
Because the conventional Ethernet switch 102 permits packets received on any link to be transmitted out over any other link(s), the switching fabric 208 and the routing logic 212 are necessarily complex. This complexity translates to an increased implementation cost for the switch. As will be appreciated, the cost of the Ethernet switch is significant since, apart from the end user devices, it is the device required in the greatest numbers for deploying the above-described access network. Accordingly, what is desired is a point-to-multipoint interface device for access networks that manages the transfer of data between a plurality of end user devices and a higher level node, yet is less expensive to implement than conventional Ethernet switches. For example, the desired point-to-multipoint interface should manage the transfer of data between a plurality of end user devices and a higher level node without requiring switching fabric and/or complex routing logic as used in conventional Ethernet switches.
Additionally, conventional Ethernet switches such as conventional Ethernet switch 102 support only identical transmit and receive rates from each port. This means that the rate at which data is transmitted to each end user device must be identical to the rate at which data is received from each end user device. However, in many access networks, the downstream bandwidth requirements are substantially higher than the upstream bandwidth requirements. This is true, for example, where the access network is used to provide broadcast video services from the service provider to the end users. In theory, this asymmetrical aspect of access networks could be exploited if the Ethernet switch permitted the use of slower, and therefore cheaper, transmitters and receivers on the upstream link from the end user devices, and higher-speed transmitters and receivers on the downstream link. Unfortunately, conventional Ethernet switches do not provide this capability. Accordingly, the desired point-to-multipoint interface device should also provide for different rates of transmission and reception from the same port, such that different upstream and downstream transmission rates may be supported over the same link.